Making his first trip of the day, the operator of Green Line car 3892 should have been rested. Instead, he had just finished moonlighting at another job, leaving him so drowsy, investigators have concluded, that he allowed his trolley to ram another last week at Boylston Street ­Station.

MBTA leaders said Wednesday that the operator was solely responsible for the rear-end crash and fired the 46-year-old operator less than a week after he was supposed to receive an award for safe driving.

The T does not ban employees from working second jobs but ­expects them to be alert and prepared, said Jonathan R. Davis, acting general manager.

“This individual failed to follow MBTA rules and policies regarding fitness for duty, and because of his failure he caused a collision that ­resulted in injuries to multiple customers, employees, and damage to MBTA property over $500,000,” ­Davis said, citing “alarming dis­regard for customer and employee safety” as grounds for termination.

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The crash required that 37 people — more than 30 passengers, plus the driver and two other MBTA employees — be taken to hospitals with what T officials deemed ­“minor to moderate injuries,” and it interrupted service on the busiest section of the Green Line for three hours.

The driver, interviewed repeat­edly by T investigators, revealed this week that he had worked from midnight to 8 a.m. at a part-time job before reporting for his full-time job with the MBTA Wednesday morning and did the same thing Thursday, the day of the crash, Davis said.

T officials declined to name the driver or describe his second job, calling his firing an ­internal personnel matter and the nature of the other job irrelevant.

The driver had worked for the T since 2006 and earned $63,398 plus benefits as a Green Line operator last year, T spokesman Joe Pesaturo said.

He received a speeding violation on an above-ground section of the Green Line in July 2009 but had a clean record since then; he was supposed to be one of 221 Green Line operators to receive a pin last Friday for safe driving.

John Lee, president of the Boston Carmen’s Union, did not respond to phone and e-mail ­requests for comment Wednesday.

Approached outside a Depart­ment of Transportation board meeting, he told a reporter: “I’m not commenting on that accident. I’ll get back to you.”

The T does not allow drivers to log more than 60 hours a week and requires them to take at least six hours off between shifts. They are subject to random drug and alcohol tests, and supervisors assess whether drivers appear fit for duty when the operators sign in each day, Pesaturo said.

The T also has a fatigue awareness program, which ­Davis said the driver had completed twice in the past year.

Officials could not say if the driver fell asleep or briefly tuned out — no video of the ­operator’s cab exists — but blamed fatigue.

The driver crashed his two-car trolley into the rear of a stopped trolley while driving about 10 to 13 miles an hour. The crash happened in the original 115-year-old core of the Green Line, the nation’s oldest subway.

Davis said officials do not know how many employees hold second jobs. While management may consider new policies in light of the crash, off-work hours are hard to control, he said.

“I’m not sure we can legislate what people do outside of the MBTA,” he said. “However, they obviously are aware that they need to be fit for duty when they come in. There is some self-responsibility that’s required.”

Davis and Richard A. Davey, the transportation secretary, each of whom commute on the Green Line, emphasized that the line is safe.

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